Reducing sugar production from spent coffee grounds using microbubble-assisted synthesis of silica acid catalyst

Abstract

The popularity of coffee as a beverage has led to a large amount of spent coffee waste worldwide, of which spent coffee ground (SCG) is one of the most heavily generated residues. Considering the environmental and social pressure to reduce pollution and increase sustainability, it is important to explore the application of SCGs in the field of renewable energy. Polysaccharides account for the most abundant components in SCGs, mainly including hemicellulose, cellulose, lignin, fat, and dietary fiber. Herein, SCG is utilized as an alternative to commercial cellulose to produce reducing sugars via hydrolysis using a sulfonated silica catalyst. Sulfonated silica catalysts are heterogeneous catalysts with advantages comprising easy separation and high recyclability. In this study, a simple and template-less method to synthesize mesoporous silica substrates by employing an in-situ microbubble generator was developed. The microbubbles generated during silica synthesis enhanced the structural properties and production yield of the silica spheres. The produced silica substrates were subjected by hydrothermal sulfonation, which resulted in a total acid density of 5.49 mmol g−1 and sulfonic acid density of 0.87 mmol g−1. The total reducing sugar yield of 31.4% was achieved by SCG hydrolysis over microbubble-assisted sulfonated silica catalysts (SSCs) at 150 ℃ for 15 h, with C6 sugars including mannose and galactose as the main products. The recyclability of SSCs was determined with a reusability test under identical coffee hydrolysis conditions. A reducing sugar yield of 96% could be maintained, which verified the high chemical structural stability of SSCs during the synthesis employing the bubble generator and hydrothermal sulfonation process. For comparison, the SSCs prepared by stirring at room temperature without a micro-bubble generator showed a relatively lower sugar yield of 24% under the same hydrolysis conditions. Therefore, this novel microbubble-assisted synthesis described herein is a promising economical, eco-friendly, and simple technique for fabricating structural silica templates as catalyst supports for biomass conversion.